Build a browser app called Swing Lab for a teacher-led pendulum investigation. Use deterministic physics, with no runtime AI calls, login or database. Use T = 2*pi*sqrt(L/g), with length in metres and gravity in m/s². Add length 0.25–4.00 m, gravity 1.62–20.00 m/s² and bob mass 0.10–2.00 kg sliders with visible units. Keep the small-angle amplitude at 8 degrees. Animate the bob with the calculated period. Changing mass must change the drawn bob size but not the period. Show period to three decimals, Pause/Resume and Reset. Ask learners to predict before changing a control, then record one observation. Include a keyboard-accessible text equivalent and a reduced-motion option. Explain the ideal small-angle model and its limits. Use teal, coral and plenty of white space. Show the working preview.